I have a 7-year old step-son who is a murder for attacking me whenever he feels like it. I found that direct iriminage works excellent. If you don't move from where you started he will fall on top of you, that way you can support him and make sure he doesn't hurt himself. For a series of fast jabs I found Tai Chi's 'sticky hands' to be a good solution. It helps you to learn blending. He sometimes climbs on my knees (so i have no chance to move away) and pretends that my torso is a punching bag. I hold his fists but make sure that my hands are totally relaxed and blend with his jabs just making sure that he misses me. it can be a lot of fun.
I wouldn't advice kokyunage if you want to avoid damage. if he is interested in Aikido, like many people here said, take him to your dojo. A common interest like this should give you both another way to bond.